1. Field of the Invention
The present invention relates to a polymer scale deposition preventive agent and a process of producing a vinyl chloride polymer by using the same.
2. Description of the Prior Art
Generally a vinyl chloride polymer is produced by polymerizing vinyl chloride monomer or a vinyl monomer mixture mainly comprised of vinyl chloride monomer in the presence of a polymerization initiator. At that time, polymer scale tends to deposit on sites in a polymerization vessel with which the vinyl chloride monomer or the monomer mixture comes in contact, such as the inner wall of the polymerization vessel and an agitation apparatus in the polymerization vessel.
The polymer scale deposition results in a decrease in the yield of the polymer and a decrease in the cooling ability of the polymerization vessel. In addition, when the deposited polymer scale comes off to mix in the resulting polymer, fish eyes are produced in formed products, such as sheets, obtained by forming the polymer, whereby the quality of the formed products is extremely lowered.
The formed product is discolored more or less even where a discoloring agent is not added. This discoloration is called initial discoloration and is desirably minimized as far as possible. Also, the initial discoloration become stronger due to the mixing of polymer scale.
To remove polymer scale from the inner wall and the like of the polymerization vessel requires excessive labor and time. The polymer scale contains unreacted monomers which are highly hazardous to the human body and therefore involves a problem of safety.
With respect to the prevention of polymer scale from depositing, conventionally, there are known a method wherein a polymerization vessel inner wall surface is coated with a polymer scale deposition preventive agent comprised of a polar organic compound, such as an amine compound,sa quinone compound, and an aldehyde compound, to form a coating film, as carried out, for example, in suspension polymerization of vinyl chloride and a method wherein such a polymer scale deposition preventive agent is added to an aqueous medium in suspension polymerization (Japanese Patent Publication (KOKOKU) No. 45-30343).
Coating with a polymer scale deposition preventive agent is generally carried out once for polymerization of one batch and in some cases once for polymerization of several batches (2 to 6 batches). Where the above polymer scale deposition preventive agent is used at this frequency, the polymer scale deposition preventive effect is exhibited until the polymerization is carried out to the extent of 5 to 6 batches, but, the preventive effect cannot be obtained at more batches. Particularly, where a water-soluble catalyst is used in polymerization, its influence is so remarkable that an industrially satisfactory result cannot be obtained.
To overcome this disadvantage, for example, a method is suggested wherein a coating film is formed by coating the inner wall surface or the like of a polymerization vessel with a polymer scale deposition preventive agent whose active ingredient is a condensation product of an aromatic amine compound with an aromatic nitro compound (Japanese Patent Publication (KOKOKU) No. 60-30681), a reaction product of a phenolic compound with an aromatic aldehyde (Japanese Pre-examination Patent Publication (KOKAI) No. 57-192414), a reaction product of a polyhydric phenol with an aliphatic aldehyde (Japanese Patent Publication (KOKOKU) No. 57-502169), or a reaction product of 1-naphthol with formaldehyde (Japanese Patent Publication (KOKOKU) No. 01-31523). Where these polymer scale deposition preventive agents are applied at the above frequency, even after the polymerization is carried out to the extent of 100 to 200 batches, polymer scale is not deposited on the liquid phase part in the polymerization vessel. The same is secured even where the above water-soluble catalyst is used.
Even the use of these polymerization scale deposition preventive agents is, however, accompanied by the defect that polymer scale is still deposited near the interface between a gas phase and a liquid phase located at the upper part of a polymerization vessel, on an agitator and the like in the polymerization vessel. The polymer scale deposited on these sites tends to grow as the polymerization is repeated, and the polymer sale may come off easily.
As means of solving these defects, there is proposed a polymer scale deposition preventive agent comprising an alkaline solution containing a condensation product of a hydroxynaphthalene compound with an aldehyde compound and an inorganic colloid (Japanese Pre-examination Patent Publication (KOKAI) No. 06-206909). The use of this polymer scale deposition preventive agent can prevent polymer scale from depositing, for example, near the interface between the gas and liquid phases.
The polymer scale deposition preventive agent is applied at such a frequency that it is applied once for polymerization of 1 to 6 batches, but where the number of batches for polymerization is increased, for example, to over 50 to 100 batches, the application is carried out more frequently.
The use of the above polymer scale deposition preventive agent increases gradually, however, the thickness of the coating film on areas including baffles, a shaft, agitating blades and the like in a polymerization vessel, and in some cases this thick coating film comes off to mix in the polymer. In particular, in the case where an inorganic colloid is added to a polymer scale deposition preventive agent, the coating film is liable to become thick and is likely peeled off.
The mixing of the peeled coating film in the polymer brings about colored foreign particles, a defect, and therefore the quality of formed products is damaged markedly.